John Koelmel, the gregarious chief executive of First Niagara Financial Group Inc., is departing immediately from the Buffalo-based financial services company, the bank said Tuesday.

In his wake, the bank’s board of directors has appointed Gary Crosby to serve as interim president and CEO. Crosby, who was hired by the bank in 2009, is currently executive vice president and chief administrative and operations officer.

A special committee of the board of directors has been formed to hire a permanent president and CEO. The committee, which will retain an executive search firm to consider internal and external candidates, will be chaired by Nathaniel Woodson.

Koelmel’s departure was “mutually agreed upon” by the board and the 60-year-old former accounting executive, the bank said Tuesday. Officials did not elaborate.

Koelmel, the highest-paid banking executive in Western New York in 2012, has been at the helm of the First Niagara Bank parent since December 2006. He has led the bank through quick and substantial growth, expanding the franchise into three new states, adding hundreds of new retail offices and increasing assets from $8 billion in to $38 billion today. The last acquisition took place in May when First Niagara bought 195 former HSBC Bank USA N.A. branches across New York state and Connecticut.

The bank paid approximately $900 million for the network. The steep price has been questioned by some banking analysts and shareholders who think it was too much. First Niagara’s stock price closed Tuesday at $8.44. It has been on a mostly downward trend for the past two years.

As part of Koelmel’s departure, he will also give up his seat on the board of directors.

The board chair on Tuesday gave credit to Koelmel for his years of leadership.

“John Koelmel has guided the company’s transformation from a local thrift to a leading Northeast banking franchise and led First Niagara during a period of difficult economic conditions and financial industry turmoil,” G. Thomas Bowers said in a release. “The board and I are grateful to John for his leadership through this critical period in our history and for positioning us so that we can focus on enhancing shareholder value through organic growth and the efficient operation of the business we have today.”

Koelmel and other bank officials have said in recent months that the bank is pulling back from its recent acquisitive mode to concentrate on the banking business it now operates. This year alone, the bank plans to trim $40 million in expenses through branch closures and layoffs in the retail sector, Scott Fisher, director of retail channels, said this month.

Koelmel said he “value(s) the opportunity” he’s had to drive the bank’s quick growth.

“I agree with the board that it’s in the best interests of the organization under present circumstances to move forward with new leadership,” he said in the release.

Crosby, 59, is a graduate of Canisius College. He is the former chief financial officer of the Buffalo Public Schools Districts and a founding shareholder of ClientLogic Corp.

First Niagara operates approximately 430 branches in New York, Pennsylvania, Connecticut and Massachusetts. It employs about 6,000 people.